如何使用DOM API访问XML节点的值

Question

I am trying to access some values in an XML document with Dom parser and I am getting a strange null pointer exception error. the code I am using is:

    import java.io.File;
    import javax.xml.parsers.DocumentBuilder;
    import javax.xml.parsers.DocumentBuilderFactory;
    import org.w3c.dom.Document;
    import org.w3c.dom.Node;

    public class readXML {

    public static void main(String[] args) {
    File file = new File("C:\\Users\\manolaki\\Desktop\\assets.xml");
    try {
      DocumentBuilder builder = DocumentBuilderFactory.newInstance().newDocumentBuilder();
      Document doc = builder.parse(file);
      doc.getDocumentElement().normalize();

            Node node = doc.getElementsByTagName("assets").item(0).getChildNodes().item(1).getChildNodes().item(1).getAttributes().getNamedItem("src");

            String boo = node.getNodeValue();

            System.out.println("Name is " + boo);

      }
    catch (Exception e) {
      e.printStackTrace();
      }
   }
}

the xml I am Using:

 <?xml version="1.0" encoding="UTF-8"?>
<xml>

<assets>
<sprite>
    <img src="menu.png"/>
</sprite>
<sprite>
    <img src="background.png"/>
</sprite>
<sprite>
    <img src="buttons.png"/>
</sprite>
<sprite>
    <img src="animals.png"/>
</sprite>
<sprite>
    <img src="cycles.png"/>
</sprite> 
<sprite>
    <img src="texts.mp3"/>
</sprite>
<sprite>
    <img src="music.mp3"/>
</sprite>
<sprite>
    <img src="click.mp3"/>
</sprite>
<sprite>
    <img src="swipe.mp3"/>
</sprite>
<sprite>
    <img src="dog.mp3"/>
</sprite>
</assets>

<assets>
<sprite>
    <img src="mehnu.png"/>
</sprite>
<sprite>
    <img src="background.png"/>
</sprite>
<sprite>
    <img src="buttons.png"/>
</sprite>
<sprite>
    <img src="animals.png"/>
</sprite>
<sprite>
    <img src="cycles.png"/>
</sprite> 
<sprite>
    <img src="texts.mp3"/>
</sprite>
<sprite>
    <img src="music.mp3"/>
</sprite>
<sprite>
    <img src="click.mp3"/>
</sprite>
<sprite>
    <img src="swipe.mp3"/>
</sprite>
<sprite>
    <img src="dog.mp3"/>
</sprite>
 </assets>

 </xml>

The code works fine as is and it prints out "menu.png" at the console. The problem is that while I can navigate through the assets nodes using the first "Item()" part, if i try to change the second "item()" to navigate through the sprites it prints a null pointer exception error. for example it works if I use:

                Node node = doc.getElementsByTagName("assets").item(1).getChildNodes().item(1).getChildNodes().item(1).getAttributes().getNamedItem("src");

but not when I use

            Node node = doc.getElementsByTagName("assets").item(0).getChildNodes().item(2).getChildNodes().item(1).getAttributes().getNamedItem("src");

Which I think is kind of strange.. given that eclipse is not finding any errors. any help would be appreciated. thanks

Answer 1

Your problem is that item(0) of the NodeList is a text node, and text nodes don't have child nodes which is where your NullPointerException is being thrown.

Try adding this to your code to verify:

Node child0 = doc.getElementsByTagName("assets").item(0).getChildNodes().item(0);
if (child0.getNodeType() == Node.TEXT_NODE) {
    System.out.println("child node is text");
}

So just keep in mind that you need to check the type of each node when iterating through a NodeList.

In your case, your odd numbered children are text nodes and your even children are elements. Try this:

NodeList nodes = doc.getElementsByTagName("assets").item(0).getChildNodes();
for ( int x = 0; x < nodes.getLength(); x++ ) {
    Node node = nodes.item(x);
    if (node.getNodeType() == Node.TEXT_NODE) {
        System.out.println(x + " text");
    } else if (node.getNodeType() == Node.ELEMENT_NODE) {
        String nodeName = node.getNodeName();

        System.out.println(x + " Node: " + nodeName);
    }
}

Output:

0 text
1 Node: sprite
2 text
3 Node: sprite
4 text
5 Node: sprite
...

You might want to check out this article from Dr. Dobb's which has some good example code from which you could create some convenience methods for low level DOM parsing like this.

Answer 2

For a simpler API check out Xsylum . Using the DOM:

String src = Xsylum.elementFor(xmlFile).get("sprite").get("img").attribute("src");

Or using XPath:

String src = Xsylum.elementFor(xmlFile).value("//sprite[1]/img/@src");